Mouse genetics has proven to be extremely valuable to biomedical research by permiting the investigator to examine the role of genes involved in a wide variety of disease processes and in validating or correcting the results of biochemical and molecular biology experiments. The ability to generate specific mutations in genes as well as conditional alleles of genes has greatly expanded the repetoire of transgenic and knockout model s available for study. To meet this ever-increasing demand for mouse models, DM ASS Medical School has an established Transgenic Animal Modeling Core. Since its inception in 1991, the mission of the Transgenic Animal Modeling Core is to produce genetically-altered mice for the UMASS Medical School community in a timely and cost-efficient manner. The size and scope of the Core was greatly expanded in 1997 and is now composed of two facilities, the Gene Targeting Facility- performing gene targeted ES cell work, and the Transgenic Mouse Facility, performing pronuclear injections, blastocyst injections, cryopreservation, and rederivation. In the last five years the Core has generated more than 250 different lines of transgenic mice, targeted over 40 genes in mouse ES cells, produced over 50 lines of gene-targeted mice via blastocyst injection, and has cryopreserved more than 35 lines of mice for UMASS researchers. In addition, the Core acts as a resource for information, protocols, and assistance for UMASS investigators working with genetically altered rodents and for those scientists performing ES cell work in their own labs. Furthermore, the Core now offers microinjection of DNA and lentiviral constructs into rat embryos in order to generate transgenic rats. Greater than 70% of all rodent models generated by the Transgenic Animal Modeling Core have been for UMASS DERC members.